This invention relates generally to the subsea burial of products such as pipelines, umbilicals and power cables and more particularly concerns the use of jetting systems to bury and protect these products in soft and loose materials, such as soft and medium stiffness clays and in sands and silts.
In most conventional jetting systems, one or more high-pressure water-jetting swords are used to excavate a trench. The jetting creates a mix of water and excavated soil and, assuming continuance of a super-critical density of the resulting mix, the product will fall by gravity to the trench floor.
However, as the mix of water and soil begins to settle, its density increases and descent of the product gradually slows. At critical density product descent ceases, often significantly before the product has reached the floor of the trench. In the sub-critical density range that follows, the settling soil solidifies under and around the stabilized product. The product never reaches its desired depth in the trench.
Also, while the pipelines, umbilicals and power cables buried using jetting system techniques do have inherent stiffness, they tend to bend under their own weight to natural minimum bend radii exceeding two meters. The greater the bending radii, the longer the time required for the product to reach the desired depth and the greater the likelihood that reaching critical density will occur before the product reaches the trench floor.
A common response to the critical density dilemma is the use of expensive, very-high-powered jetting systems, consuming as much as two megawatts of power, in an effort to increase the speed of advance of the jetting system along the product path, allowing the product to fall to the trench bottom more rapidly. This is somewhat palatable given that increased trenching speeds reduce total trenching time. But, while maximum trenchers speeds are desirable, there are many factors which, alone or in concert, limit the possibilities of increasing, and may even result in decreasing, trenching speeds in any specific application.
Furthermore, in hard soils and gravels, the jets take significant time to do their work. En route variations in the soil quality, such as mixed soils with different super-critical and sub-critical properties, soils that are both horizontally and vertically stratified, changes in the types of soil and competent soils supporting the products ahead of the jetting swords, all complicate maximizing trenching speed. Maximum speeds of the trencher tracks, the power available to the tracks and the water power available to the system all cap the possible trenching speed. For any or all of these reasons, achievement of sufficient speed to permit backfill at super-critical density cannot be assured with known jetting systems.
Known alternatives to the increasing-trenching-speed solution include the use of multiple passes of the jet system to lower the product in stages, the use of suction devices to remove the water and soil mix from the trench and directing some of the jets of the swords backwards to keep the water and soil mix at as low a density as possible. Multi-pass systems increase time and cost. Adding devices increases cost and complexity. Redirecting jets diminishes the cutting forces applied by the system and slows progress along the product path.
Other problems with presently known jetting systems include their mass which is typically in a range of 15,000 kg and requires sophisticated launch and recovery equipment, their high sensitivity to weather, their reliance on delicate equipment which makes repair difficult and time consuming, and their multiple lift lines, hoses and control umbilicals which can lead to entanglement with, and loss of control of, the trencher.
It is, therefore, an object of this invention to provide a jetting system which maintains the water and soil mix at a super-critical density for longer distances behind the jetting swords. Another object of this invention is to provide a jetting system which facilitates rapid descent of the product in the trench. It is also an object of this invention to provide a jetting system which increases the likelihood of the product reaching its desired depth in the trench. A further object of this invention is to provide a jetting system which permits the advance of the jetting system along the product path at lower speeds. And it is an object of this invention to provide a jetting system which reduces the need for multiple passes of the jetting system to lower the product in stages, suction devices to remove the water and soil mix from the trench and/or redirection of some of the jets of the swords backwards to keep the water and soil mix at as low a density as possible.